Magnetic Resonance Imaging (MRI) is emerging as the modality of choice for soft-tissue tumor visualization in image-guided therapeutic procedures such as surgery, mild therapeutic hyperthermia, high-intensity focused ultrasound (HIFU), microbubble radiosensitization, and most notably, radiation therapy for the treatment of cancer. This is due to its superior soft-tissue image contrast compared to other mainstream imaging modalities such as ultrasound (US) and x-ray based imaging, i.e. computed tomography (CT) and cone beam CT (CBCT). Radiation therapy, which aims to kill cancerous tumors by delivering high doses of radiation with high accuracy (1-2 mm), relies heavily on imaging for patient diagnosis and daily treatment setup verification. In spite of MRI's flexibility in providing multiple contrast mechanisms (e.g., T1/T2-weighted imaging) and multiplanar acquisition modes, MRI's use is limited, being typically relied upon only as an additional (sometimes elective) imaging session to complement CT-derived data. The main reasons for this are that MR images lack information regarding tissue electron densities (or CT numbers), which are required for treatment plan dose simulations and most important, MR images are intrinsically affected by geometric image distortions.
MR image distortions are commonly classified in two main categories: a) system or scanner related distortions due to inhomogeneities in the MR's main magnetic field and nonlinearities in the imaging gradients, and b) object or patient-induced distortions caused by local variations of tissue magnetic susceptibility properties and chemical shift effects. The system-related distortions are in the range of 1-2 cm over a large field of view (FOV), i.e. pelvis, and gradually increase in magnitude with distance from the MR's isocenter. In contrast, the magnitude of patient-induced distortions is less significant, i.e. in the range of a few mm, and localized at the interface between various anatomical structures.
MRI is expected to play an increasing role in radiation therapy with the advent of newly emerging MR-guided technologies such as MR-linac and MR-60Co systems. MR-linac systems refer to the integration of an MR scanner with a therapy linear accelerator (linac) such that MR imaging is available in the therapy room. The current proposed MR-linac designs refer to a) coupled systems in which both the MR scanner and the linac are mounted on a single gantry to facilitate treatment delivery from any rotational angle and b) a proximity system in which the MR scanner is on-rails and travels in and out of the linac vault and can image at a predefined location in close proximity of the linac. Similar to the MR-linac systems, the MR-60Co combines an MR scanner with three 60Co sources mounted on the same gantry (e.g. ViewRay, Oakwood Village, Ohio). The 60Co sources can rotate around the patient to deliver radiation from multiple positions.
For MRI-guided radiation therapy, MRI is expected to become the primary imaging modality for generating image data used for both treatment simulation and daily patient setup verification. Quantification and correction of image distortions is preferably done before the images are integrated into the treatment planning process. The MR images should exhibit a spatial accuracy similar to CT. Uncorrected geometric errors are likely to reduce the accuracy of the treatment plan simulation and delivery, which ultimately leads to poor patient treatment outcomes.